High performance heat exchangers

Abstract

Disclosed are means for improving the service-life of indirect tubesheet type heat exchangers used in chemical reactors, particularly those exposed to reducing, nitridizing and/or carburizing environments. Such means include the use of certain ferrules within the heat exchange tubes and/or weld types used in construction of these heat exchangers.

1. A process for preparing hydrogen cyanide comprising the steps of: feeding reaction gas to a reactor, the reaction gas comprising hydrocarbon, ammonia and optionally an oxygen-containing gas; reacting the reaction gas in the presence of a catalyst to give product gas; cooling the product gas in a heat exchange apparatus comprising (a) a shell having an entry tubesheet and an exit tubesheet, each tubesheet having a plurality of holes, wherein the shell has at least one inlet and one outlet for heat exchange medium; (b) a plurality of tubes disposed within the shell wherein an entry end of each tube is affixed to the entry tubesheet and an exit end of each tube is affixed to the exit tubesheet such that an axis of the tube and an axis of an entry and exit tubesheet hole are coincident; and (c) a plurality of ferrules, each ferrule having an entry end and an exit end extending through an entry tubesheet hole into a tube wherein the exit end extends below the entry tubesheet, the ferrule comprising silicon nitride; and recovering hydrogen cyanide from the cooled product gas. 
     
     
       2. The process of  claim 1  in which said entry end of each ferrule has a diameter greater than said tube; said diameter decreases in a first portion of each ferrule; in a second portion of each ferrule, an internal surface of each ferrule diverges toward said exit end at an angle of less than 30 degrees; and an external diameter of said exit end is smaller than that of said tube. 
     
     
       3. The process of  claim 2  in which each tube is formed of a metal including carbon-steel or a nickel-chromium alloy, wherein each of said plurality of holes in said entry tubesheet comprises a first bore having a first diameter, said first bore extending partially through said entry tubesheet, and a second bore having a second diameter, said second bore extending from said first bore through said entry tubesheet to a tube-side face of said entry tubesheet, said first and second bores being coaxial, said first diameter being greater than said second diameter, said second diameter being greater than an outside diameter of a corresponding tube received therein, each tube being inserted into a corresponding second bore a distance equal to an axial length of said second bore, each tube being affixed to said second bore by a full-penetration weld formed between each tube and the tubesheet over said axial length of said second bore. 
     
     
       4. The process of  claim 3  in which each tube is formed of a metal including a nickel-chromium alloy. 
     
     
       5. The process of  claim 2  in which said angle is between 5 and 7 degrees. 
     
     
       6. The process of  claim 2  in which the product gas is at a temperature of at least 1000° C. 
     
     
       7. A process for cooling a reducing, carburizing or nitridizing gas; said process comprising cooling said gas in a heat exchange apparatus comprising (a) a shell having an entry tubesheet and an exit tubesheet, each tubesheet having a plurality of holes, wherein the shell has at least one inlet and one outlet for heat exchange medium; (b) a plurality of tubes disposed within the shell wherein an entry end of each tube is affixed to the entry tubesheet and an exit end of each tube is affixed to the exit tubesheet such that an axis of the tube and an axis of an entry and exit tubesheet hole are coincident; and (c) a plurality of ferrules, each ferrule having an entry end and an exit end extending through an entry tubesheet hole into a tube wherein the exit end extends below the entry tubesheet, the ferrule comprising silicon nitride. 
     
     
       8. The process of  claim 7  in which said reducing, carburizing or nitridizing gas is a carburizing or nitridizing gas. 
     
     
       9. The process of  claim 8  in which said entry end of each ferrule has a diameter greater than said tube; said diameter decreases in a first portion of each ferrule; in a second portion of each ferrule, an internal surface of each ferrule diverges toward said exit end at an angle of less than 30 degrees; and an external diameter of said exit end is smaller than that of said tube. 
     
     
       10. The process of  claim 9  in which each tube is formed of a metal including carbon-steel or a nickel-chromium alloy, wherein each of said plurality of holes in said entry tubesheet comprises a first bore having a first diameter, said first bore extending partially through said entry tubesheet, and a second bore having a second diameter, said second bore extending from said first bore through said entry tubesheet to a tube-side face of said entry tubesheet, said first and second bores being coaxial, said first diameter being greater than said second diameter, said second diameter being greater than an outside diameter of a corresponding tube received therein, each tube being inserted into a corresponding second bore a distance equal to an axial length of said second bore, each tube being affixed to said second bore by a full-penetration weld formed between each tube and the tubesheet over said axial length of said second bore. 
     
     
       11. The process of  claim 7  in which said reducing, carburizing or nitridizing gas is a product of a steam-methane reformer.